FIG. 12 is a plan view showing a construction of a conventional rotary joint shown in JP 56-51522 B for example. In FIG. 12, reference numerals 101 and 102 respectively designate circular waveguides which are nearly identical in cross sectional size to each other and to which an interval axis is nearly common; reference numeral 103 designates a choke groove which is formed in a flange portion of a connection surface between the circular waveguides 101 and 102; reference numeral 104 designates a bearing; reference numeral 105 designates a connection portion consisting of the choke groove and the bearing; reference numerals 106 and 107 respectively designate projection portions for conversion from a linearly polarized wave to a circularly polarized wave; reference numerals 108 and 109 respectively designate input rectangular waveguides; reference numerals 110 and 111 respectively designate output rectangular waveguides; reference numerals 112 and 113 respectively designate short-circuit plates; and reference numerals 114 to 117 respectively designate coupling holes.
The choke groove 103 is the means which is usually used so that a gap defined between the circular waveguides 101 and 102 becomes equivalently short-circuit in a frequency of an electric wave propagated through the circular waveguides 101 and 102. The circular waveguides 101 and 102 are connected to each other in terms of a high frequency by a function of the connection portion 105 having the choke groove 105 while keeping a predetermined gap therebetween. The circular waveguide 102 can be rotated about a tube axis with respect to the circular waveguide 102 by a predetermined angle of rotation by a function of the bearing 104 while keeping the tube axis so that the circular waveguides 101 and 102 are aligned with each other through the tube axis.
The position of the projection portion 106 for conversion from a linearly polarized wave to a circularly polarized wave is set to the position making an angle of +45 degrees or −45 degrees with a direction of an electric field of a TE10-mode of the input rectangular waveguide 108. At this time, the position of the projection portion 107 for conversion from a linearly polarized wave to a circularly polarized wave is set to the position which, for the former, makes an angle of −45 degrees with a direction of an electric field of a TE10-mode of the output rectangular waveguide 110, and which, for the latter, makes an angle of +45 degrees. The coupling holes 114 and 116 are formed by cutting off parts of the short-circuit plates 112 and 113, respectively. The coupling holes 115 and 117 are formed by cutting off parts of sidewalls of the circular waveguides 101 and 102, respectively.
Next, operation will hereinbelow be described. After an electric wave of a TE10-mode made incident from the input rectangular waveguide 108 has been efficiently converted into the electric wave of a TE11-mode in the circular waveguide 101 through the coupling hole 114 now, it is then converted from the linearly polarized wave into the circularly polarized wave by the projection portion 106 for conversion from a linearly polarized wave into a circularly polarized wave. The circularly polarized wave obtained through the conversion is transmitted to the circular waveguide 102 through the connection portion 105 irrespective of an angle of rotation of the circular waveguide 102 due to the rotation symmetry of the mode to be guided into the output rectangular waveguide 110 through a course reverse to the above-mentioned course. That is to say, after the electric wave has been converted from the circularly polarized wave into the linearly polarized wave by the projection portion 107 for conversion from a linearly polarized wave into a circularly polarized wave in the circular waveguide 102, it is then transmitted to the output rectangular waveguide 110 through the coupling hole 116.
On the other hand, other electric waves of a TE10-mode made incident from the input rectangular waveguide 109 is efficiently converted into the electric wave of a TE11-mode in the circular waveguide 101 through the coupling hole 115. At this time, a direction of the electric field of the TE11-mode obtained through the conversion perpendicularly intersects that of the TE11-mode due to the incident wave from the input rectangular waveguide 108. For this reason, the electric wave of the TE11-mode obtained through the conversion via the coupling hole 115 is converted into a circularly polarized wave having rotation reverse to that of the TE11-mode through the coupling hole 114 by the projection portion 106 for conversion from a linearly polarized wave into a circularly polarized wave. At this time, the circularly polarized wave obtained through the conversion is transmitted to the circular waveguide 102 through the connection portion 105 irrespective of an angle of rotation of the circular waveguide 102 due to the rotation symmetry of the mode to be guided to the output rectangular waveguide 111 through a course reverse to the above-mentioned course. That is to say, after the electric wave has been converted from the circularly polarized wave into the linearly polarized wave by the projection portion 107 for conversion from a linearly polarized wave into a circularly polarized wave in the circular waveguide 102, it is then transmitted to the output rectangular waveguide 111 through the coupling hole 117.
As described above, in the conventional rotary joint shown in FIG. 12, a signal within the input rectangular waveguide 108, and a signal within the input rectangular waveguide 109 are respectively guided to the output rectangular waveguide 110 and the output rectangular waveguide 111 irrespective of presence or absence of the rotation of the circular waveguide 102 and the output rectangular waveguide 110. That is to say, the conventional rotary joint has a function as a two-channel rotary joint which is capable of transmitting different two signals at the same time.
In the conventional rotary joint, for obtaining a circularly polarized wave having excellent axial ratio characteristics, the projection portions 106 and 107 for conversion from a linearly polarized wave into a circularly polarized wave need to be provided so as to be relatively long. Thus, there is encountered a problem in that the total length becomes long.
In addition, in general, in the projection portions 106 and 107 for conversion from a linearly polarized wave into a circularly polarized wave, a frequency range in which a circularly polarized wave with excellent axial ratio characteristics is obtained is relatively narrow. Thus, there is encountered a problem in that the excellent axial ratio characteristics of a broad band are difficult to be obtained for a rotary joint as well.
The present invention has been made in order to solve the above-mentioned problems, ant it is, therefore, an object of the present invention to provide a rotary joint which is of a thin type and has broad band characteristics and which is low in loss and is excellent in power resistance.